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DAY 1: Chalk Groundwater Resources

Session 1: Geology 9:45 Andy Farrant, Mark Woods, Rhian Kendall, Rowan Vernon, Jo Thompson, Catherine Cripps, Keith Westhead, Andrew Newell, Nigel Hoad, Rolf Farrell, Edward Wrathmell, Michael Kehinde, Michael Jones. BGS, Environment Agency, Thames Water.

arf@bgs.ac.uk

3D geological modelling of the Chalk for groundwater management

The pressure on the Chalk aquifer, both in terms of groundwater quality and availability is increasing. Water quality is impacted by point source and diffuse contamination including nitrates and pesticides. Groundwater resource are impacted by the demands of an increasing population; regulation requiring the need to reduce abstraction to maintain stream flow, and the impacts of climate change leading to increased risk of drought and groundwater flooding. These all necessitate improved aquifer management and better groundwater models. To do this requires a better understanding of the host rock, particularly its spatial and stratigraphical variability. The introduction of a high-resolution stratigraphy for the Chalk in the 1990’s and its application to new geological maps has revolutionized our understanding of the geological basis for the hydrogeological and engineering properties of the Chalk. Lithology, stratigraphical discontinuities (marl seams, flints and hardgrounds) and tectonic structures (faults, joint style and folds) all influence groundwater flow. The application of the modern stratigraphy also highlights previously unrecognized tectonic structures. As such, the Chalk can no longer be treated as a single homogenous medium. The development of 3D geological modelling software now means it is possible to create high resolution 3D geological models of the Chalk aquifer.

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10:05 Lou Maurice, S. Loveless, B. Brauns, J. Bucknall, C. Manning, H. Verman, A. Leggatt, I. Karapanos, Gillian Davies, Craig Hampton, Andy Farrant, Tim Atkinson.

BGS, Portsmouth Water, Affinity Water, Environment Agency, University College London.

loma@bgs.ac.uk

Caves, stream sinks and tracer tests in the English Chalk

Rapid karstic flow in the English Chalk has been known since the first tracer tests at the start of the

twentieth century. However, evidence for karst is localised and there is still no overall conceptual

understanding of Chalk karst hydrogeology. Recent collation of data as part of a knowledge-

exchange initiative is starting to provide a national scale picture of the nature and extent of karst

development in the Chalk. Karstic caves are generally rare in the Chalk, but there are caves exposed

within the base of stream sinks; caves have been intercepted during quarrying and the construction of

wells and adits; and caves with features indicating karstic origins are present in coastal areas.

Smaller scale solutional karstic voids (solutionally enlarged fissures and conduits that are too small for

humans to enter) appear to be very common and impact on recharge, storage, flow and contaminant

transport in the Chalk aquifer. Tracer tests have demonstrated rapid groundwater flow through the

unsaturated zone and the saturated zone; both via surface karst features such as stream sinks, but

also in areas where surface karst is absent. Karstic flowpaths are classically associated with linkages

between stream sink input points at the surface and spring outlets, and such pathways occur in some

areas of the Chalk. However there is also evidence to suggest that the fissures and conduits

intercepted by Chalk boreholes are part of an extensive network of karstic voids through which rapid

groundwater flow occurs. Karst data will be presented and the implications for understanding Chalk

hydrogeology and contaminant transport will be discussed.

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Session 2: Flow Processes KEYNOTE: 10:25 John Bloomfield. BGS.

jpb@bgs.ac.uk

Hydrogeological conceptualisation and spatial variation in the Chalk: a data-led approach

Drawing on an understanding of the geology and rock mass properties of the Chalk, hydrogeological

conceptualisations of the aquifer abound in the literature and are a necessary approach to a

reductionist view of the hydrogeology. For example, conceptual models are used to inform the design

of field sampling campaigns and hydraulic tests, as well as interpretation of the results of such

investigations. They are also, typically, an important first step when modelling flow and/or transport in

the Chalk. Groundwater levels, our most widely available data on the Chalk, reflect a catchment-

integrated response of a heterogeneous aquifer to metastable meteorological forcing, and are usually

analysed by applying prior hydrogeological conceptualisations and a deterministic approach.

However, data-led analysis and modelling is increasingly practical given the growing ease of access

to larger groundwater level datasets and complementary hydrometeorological data. Here we describe

how a data-led analysis of Chalk groundwater level hydrographs can provide insights into the

hydrogeology of the aquifer. We will briefly discuss approaches to hydrograph standardisation before

reviewing the findings of five recent studies of: relationships between groundwater levels and driving

climatology; analysis of groundwater level extremes; the effects of superficial deposits on Chalk

hydrographs; and analysis of spatial variations in Chalk groundwater level at the regional and national

scales.

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10:55 Adrian Butler. Imperial College.

a.butler@imperial.ac.uk

Recharge mechanisms in the Chalk and their role in groundwater resources and groundwater flooding With its fractured, dual porosity and dual permeability matrix, the Cretaceous Chalk is a unique and fascinating aquifer. Additional complexity is provided by the inclusion of layers of flints, hardgrounds and marls, along with other vertical and horizontal heterogeneous features. Its extensive coverage over eastern and southern England means that it is the UK’s primary aquifer and therefore a key water resource, particularly during drought. The fractures, whose presence make it a highly productive aquifer, also mean that it has a low specific yield and consequently is a relatively poor aquifer in regard to active (or dynamic) storage. Indeed, this low dynamic storage makes it vulnerable to groundwater flooding, a feature that has been an episodic occurrence during the first two decades of the 21st century, with commensurate impacts on housing and transport infrastructure. How the aquifer responds during drought and flood is largely controlled by the movement of water in the unsaturated (or vadose) zone, and this, in turn, is governed by the hydraulic properties of the factures and matrix (Butler et al., 2012). Whilst there is little storage in the fractures, as Lewis et al. (1993) showed, there is substantial storage in the unsaturated zone. This storage helps explain why chalk aquifers can be so responsive during prolonged periods of high rainfall. Furthermore, the permeability of the chalk matrix means that slow drainage of water stored in the unsaturated zone can be an important recharge mechanism even during severe drought. In order to explore and quantify this effect, work previously undertaken by Lewis et al. (1993) on the behaviour of the Chalk in the Lambourn catchment, Berkshire, UK, during the summer of 1976 has been reanalysed. This has indicated that even during August 1975, at the height of one of the UK’s most intense droughts, there was downward drainage of around 0.5 – 1.0 mm/day providing recharge to the aquifer and thereby supporting flow in the river Lambourn, and hence the Thames. These results are supported by recharge estimates obtained from simulations of the adjacent Pang catchment using a physically representative Chalk hillslope model conditioned on highly detailed experimental data. The results have important implications for water resource assessment and groundwater modelling of the Chalk of southern England during both drought and flood conditions.

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11:45 Jared West, Noelle Odling, Simon Bottrell, Maxine Zaidman, Sam Allshorn, Dawn Keim. University of Leeds, JBA.

J.west@see.leeds.ac.uk

Evidence for karstic behaviour in Chalk aquifer unsaturated zones

Cretaceous Chalk aquifers frequently have very thick unsaturated zones, which control both their

hydraulic response to rainfall and the extent to which pollutants are delayed or attenuated before

reaching groundwater. Understanding unsaturated zone flow is vital for prediction of responses to

extreme events and future groundwater quality. Several University of Leeds projects have addressed

this issue. Zaidman et al., (1999, QJEGH) used soil zone hydrological monitoring and time lapse ERT

imaging of the deeper unsaturated zone, to show that recharge reached depths of 15-25m depth

rapidly, via flow along inclined joints. Allshorn et al. (2007, Geol Soc SP) introduced tracer at the

base of the soil zone and detected in a subsurface tunnel, and confirmed rapid transport occurred to

depths of up to 40m, and that tracer migrated laterally for tens of metres within the unsaturated zone.

Breakthrough curves were similar to those seen in classic karst aquifers, and modelling suggested

fracture apertures of >1mm indicative of solutional enhancement. Keim et al (2012, VZJ) monitored

discharge into the same tunnel while simultaneously monitoring soil water dynamics. Major seeps

emerged every few hundred metres with dry sections between, showing that flow pathways converge

in the unsaturated zone, possibly due to karstic development along flint-rich layers. In the recharge

period (December – May), drainage fluxes from the base of the soil zone closely matched tunnel

discharge responses, confirming a rapid ‘karstic’ responses to recharge. However, substantial

continued discharge through the summer months suggests that the karstic network has significant

storage capacity.

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12:05 Ronald Vernes, Jan Hummelman, Armin Menkovic, Reinder Reindersma. TNO

ronald.vernes@tno.nl

Estimating the hydraulic conductivity and transmissivity of the chalk aquifer in South-Limburg, the Netherlands

Thick layers of unconsolidated Cenozoic deposits are present in large parts of the Netherlands. These

deposits form important aquifers and aquitards and have been the main focus of hydrogeological

research. In the eastern and south-eastern part of the Netherlands however, consolidated Mesozoic

deposits surface. In South-Limburg these deposits partly consist of Palaeocene and Late Cretaceous

chalk, the main aquifer in this area, the properties of which are only poorly understood.

For the Dutch 3D hydrogeological model REGIS II the geometry (depth of the top and base and the

thickness) of the three geological formations that build up the chalk aquifer were modelled based on

borehole data. Estimates of the hydraulic conductivity and transmissivity of the chalk aquifer were

also made. Exposures of the chalk, especially in quarries, show that the upper part of the chalk is

karstified. This together with flow measurements in wells have led to the assumption that hydraulic

conductivity is high at the top and that it rapidly decreases with depth. Five zones of varying

thicknesses were distinguished with hydraulic conductivities ranging from 100 to 0.01 m/d. The

current approach however tends to overestimate hydraulic conductivity, especially in the northern part

of South-Limburg where the chalk aquifer is covered by unconsolidated Cenozoic deposits. A better

understanding of the karstified zone is needed in order to improve the estimate of the hydraulic

conductivity.

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12:05 Alessandro Marsili, Christopher Ford, Ilias Karapanos, Mahmoud Jaweesh, Daniel Yarker, Rob Sage. Affinity Water Ltd, Wet Rock Water Resources Ltd.

alessandro.marsili@affinitywater.co.uk

Artesian conditions in the Chalk aquifer and implications in groundwater-surface water interactions across catchments

Artesian conditions in the Chalk aquifer are currently observed in several places across different

catchments in the Chiltern Hills. Although their presence has been historically recorded since the

1800s, information relating to the borehole design is scarce and the reasons behind the groundwater

overpressure are uncertain. The artesian overflows are dependent on the regional groundwater levels

and therefore variable and intermittent. Similarities in terms of the presence of artesian boreholes and

their spatial distribution have been identified across several catchments, where their alignments seem

to follow the main Chalk strike. In particular, they appear in the Chalk sequence correspondent to the

outcrop of the Middle Chalk section. The classification of the historic and current artesian boreholes

has been investigated in order to explore the links between the aquifer properties and the geological

features of the Chalk. Attention has also been focused on the importance of the artesian boreholes in

terms of groundwater-surface water interaction, their historic use and implications for the surface

water flow in the Chalk streams. Water quality analyses have provided evidence of the local low

hydraulic connectivity between the river and the aquifer.

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Session 3: Regulation/Groundwater Management KEYNOTE: 13:45 Hans Jørgen Henriksen, Bertel Nilsson.

Geological Survey of Denmark and Greenland, GEUS

hjh@geus.dk

Mapping and modelling chalk aquifers of Denmark - groundwater protection and sustainable groundwater abstraction

Denmark (43,000 km2, 5.6 million inhabitants) rely entirely on groundwater for water supply. Main

challenges are groundwater contamination from diffuse and point sources, intensive abstraction and

climate change effects. GEUS support Danish EPA (MST) with the development of a National Water

Resource Model (MIKE SHE 500x500 m, www.vandmodel.dk) for quantitative status assessment of

groundwater bodies. A third of the total water supply for drinking water in Denmark is abstracted from

Chalk and limestone aquifers, and these aquifers are often vulnerable to land use activities. Seven

out of nine limestone groundwater bodies are at risk or with poor quantitative status.

Chalk and limestone aquifers have complex hydraulic characteristics that are highly variable, with

fractures enhanced by solution. This raise concerns about current mapping and modelling, where

chalk and limestone normally is handled as equivalent porous medium (EPM), using standard

groundwater flow codes, which obviously is not always a valid approximation. Mapping of conceptual

model flow system and handling of parameter and model structure uncertainty in the modelling

process require more knowledge about chalk and limestone. Especially the effective porosity, key for

administration of wellfield protection zones, is disputed. Estimates range from below 1 % to above 20

%. The presentation gives examples of mapping and modelling and discuss whether limestone

aquifers in Denmark could be more karstic than previously evaluated. In Northern Jutland surface

karst features (sinkholes) has be mapped with 40 – 160 cm DTM, but the question is to which degree

karst groundwater circulation is taking place in chalk and limestone areas without any karst surface

features.

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14:15 Mike Packman

Southern Water

mike.packman@southernwater.co.uk

Sustainable Chalk Groundwater Resource Planning and Management. Hydrogeology is the basis of Chalk groundwater resource planning and management. Up until the late

1980s Chalk abstraction licencing in England and Wales primarily involved assessment of possible

derogation of other abstractor’s rights, with little regard to environmental impact. In the 1970s/80s Chalk

hydrogeology was mainly about quantity and then quality considerations came to the fore in the 1990s.

Now groundwater resource planning is dominated by environmental considerations and clawing back

licenced quantities to make abstractions more sustainable. Hydrogeology plays a significant role in

water resource planning and management at Southern Water, as over 70% of our public water supply,

(PWS), comes from groundwater, of which over 90% comes from the Chalk aquifer. Two of our largest

surface water abstractions are in Hampshire and are from the Rivers Test and Itchen with Base Flow

Indices of 0.95 and 0.99 respectively, so a MODFLOW groundwater model is used for flow simulation.

Southern Water have developed detailed hydrogeological conceptual models that allow state-of-the-art

numerical modelling of complex well and adit system water supply works and environmentally sensitive

sites in the Brighton and Worthing Chalk blocks, using refined grids and coupled linear nodes. As we

can now accurately simulate rest and pumped groundwater levels and flows in both PWS sources and

sensitive sites, we are able to model time series of source outputs and environmental impacts in

extreme drought and climate change scenarios using stochastically generated rainfall records. We are

now constructing Chalk groundwater source and environmentally sensitive site models for the Isle of

Wight and Kent.

.

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14:35 Sally Watson, Alison Carruthers, Jane Dottridge, Megan Durrant, Charles Jones, Matt Rickard Mott MacDonald, HS2

sally.watson@mottmac.com

New insights into Chalk hydrogeology near Wendover – HS2 investigations and conceptualisation. High Speed 2 (HS2) Phase One is the new high speed rail link between London and Birmingham,

which will cross the wide chalk outcrop in the Chilterns. Springs originating from the chalk are located

within 1km of the line of the route, and supply significant quantities of water to support flow to the

Wendover Branch of the Grand Union Canal. To the north-west of Wendover the HS2 route is located

in a cutting which has the potential to impact on groundwater by intersecting the water table. As part

of the Parliamentary approval process (known as a hybrid Bill), the hydrogeology around Wendover,

on the chalk scarp slope of the Chilterns, has been subject to critical review, due to concerns about

the possible impacts of the scheme on the springs in the area. The hybrid Bill process to gain

approval for HS2 to proceed to detailed design and construction required that an appropriate level of

assessment of potential environmental effects of the proposed route is undertaken. This assessment

is derived from existing information. In addition to the existing data, a programme of ground

investigations was undertaken which collected additional information to support the evidence on the

geological and hydrogeological data in the area. This provided updated information on the

hydrogeological setting of the line of the route in this location. The talk will discuss:· the Parliamentary

process for major infrastructure and the typical hydrogeological inputs, the level of detail required, the

datasets used and the post authorisation roles for Regulators and other Authorities;· the source-

pathway-receptor model for the Wendover area, considering the springs as the most important

receptors of potential impacts; and an updated conceptual model for the chalk hydrogeology around

Wendover and the additional information provided by the ground investigations linking Professor.

Rory Mortimore’s geological interpretation along the line of the route to the existing conceptual

hydrogeological model, with emphasis on the main flow horizons supplying the springs. The outcome

of the ground investigations resulted in an updated conceptual hydrogeological model with an

improved understanding of the geological controls on spring flow. The information provided additional

evidence to the analysis that impacts from the cutting on the spring flows are expected to be

insignificant in terms of flow and water quality.

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14:55 Ilias Karapanos, Rob Sage.

Affinity Water Ltd, Wet Rock Water Resources Ltd.

ilias.karapanos@affinitywater.co.uk

The Chiltern Chalk Streams and Groundwater Abstraction – Can this relationship survive? The Chilterns Chalk is currently represented as a single layer aquifer in the Modflow groundwater

models used by the Environment Agency, with the addition of varying vertical and horizontal hydraulic

conductivity to allow for heterogeneity. This supports the assumption that the groundwater abstracted

is at the expense of river flows under all background groundwater level conditions. A case study in the

River Ver catchment is presented, using water level measurements in observation boreholes installed

at different depths in the Chalk aquifer and compared to river flow data. Following changes in

abstraction patterns under different background groundwater level conditions, it is clear that there is

dual piezometry in the Chalk aquifer in at least two locations. Under abstraction influence, there are

two water levels in the Chalk, one in the lower Chalk units corresponding very well to abstraction

changes and one in the higher Chalk units showing some response but sustaining up to a 4m

difference in water levels. This difference between the two Chalk water levels increases in a drought,

resulting in dewatering of the lower chalk units but not at the expense of the higher Chalk

groundwater levels or river flows. The authors suggest that there is geological control in the New Pit

Chalk units with the marl seams and the hard bands controlling the leakage between the various

Chalk layers under abstraction influence. Further targeted investigation will prove the spatial extent of

this, aiming to change the conceptual model of how the Chilterns Chalk needs to be simulated.

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15:45 Rik Ingram, Richard Morgan, Susie Roy, Rob Soley.

Wood, Environment Agency.

rik.ingram2@woodplc.com

Falling abstraction, rising groundwater: exploring the confines of the Lincolnshire Chalk Aquifer. Grimsby has an important and changing relationship with groundwater. The main aquifer underlying

the town is the Chalk and the karstic characteristics of this aquifer complicate groundwater

management. The Chalk here is confined by 12-18 m of glacial till but artesian springs – ‘blow wells’

– are associated with permeable pathways through the superficial cover. Offshore from Grimsby the

glacial deposits have been eroded and the Chalk is exposed. The presence of natural discharge

points has resulted in the development of a preferential flow path between the Chalk outcrop on the

Lincolnshire Wolds and the coast at Grimsby. Favourable aquifer properties enabled groundwater

abstraction and by the mid-late 20th Century, saline intrusion was affecting groundwater quality.

Management of that issue combined with other pressures has resulted in a reduction in groundwater

abstraction and rising groundwater levels. Groundwater flooding currently has direct effects on

property as well as inundating drainage and potentially affecting bathing water quality. The feasibility

of using groundwater abstraction to mitigate groundwater flooding has been explored using a

groundwater model. The model gives a clear account of the history of groundwater flooding, showing

the link between reduced abstraction and rising groundwater levels. Reductions in the frequency and

duration of groundwater flooding can be accomplished by increasing abstraction within existing yield

and licence constraints. However, this must be accomplished sensitively. ‘Smart’ approaches to

groundwater abstraction could be used to pump more water only at times of higher groundwater level

(approximately 30% of the time). This minimises effects on Chalk streams and saline intrusion.

Innovative solutions are required to make best use of this seasonally available groundwater and to

accommodate the large increases in pumping rate.

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Session 4: Extreme Events and Engineering in the Chalk

16:05 Mike Streetly, Sam Bishop, David Bradley, Rick Hayes, Daniel Clark. ESi, APEM, Cambridge Water.

mikestreetly@esiconsulting.co.uk

Managing public water supply abstraction from a Chalk aquifer to minimise risk of deterioration of WFD status.

The Cam and Ely Ouse Chalk aquifer has been an important source of public water supply for over

100 years. In response to growing demand for water, the National Rivers Authority developed the

Lodes-Granta scheme to provide augmentation water to key rivers affected by groundwater

abstraction. However, during the droughts in the late 1990s, the River Granta, which derives

baseflow from the Chalk aquifer, was dry in places for several months. In response, the Environment

Agency and Cambridge (South Staffs) Water carried out investigations into the impacts of abstraction

on the flow and ecology of the Granta and options for applying Hands Off Flow conditions to the

abstraction licences of operational sources were agreed. However, these restrictions would result in

a shortfall within the relevant supply zone in some conditions and so Cambridge Water is seeking to

increase abstraction from an alternative source of groundwater within the catchment. The

Environment Agency was concerned that this abstraction could pose a risk of deterioration of

ecological status of the water body under the Water Framework Directive. This paper describes the

investigations undertaken to assess the risk of deterioration and shows how these are being used to

manage this risk going forward.

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16:25 Steve Worthington, Rob Soley.

Worthington Groundwater, Wood.

sw@worthingtongroundwater.com

Is turbulent flow a major factor in groundwater flooding in the Chalk?

The Chalk aquifer in England has both seasonal and short-term responses to precipitation. The latter

response can result in the water table rising to the surface in dry valleys, causing groundwater

flooding. This has been a serious problem in some areas in the Chalk, and has attracted substantial

attention since extensive groundwater flooding in 2000-2001. Groundwater levels can rises up to

several metres per day following heavy rain, and it has been a challenge to explain such rises. We

examined the possibility that turbulent flow might be a factor in the large and rapid increases in

groundwater levels. Head is proportional to discharge with laminar flow, but it is proportional to the

square of discharge when flow becomes turbulent. The unconfined Chalk aquifer to the north of

Havant drains to a group of springs with a combined discharge of 95 Ml/d. The normally dry valleys at

Hambledon and Rowlands Castle experience groundwater flooding, and water levels in wells close to

these valleys were correlated with spring discharge. There was a marked break in slope on both plots,

and this is compatible with a change from laminar to turbulent flow. Calculated fissure apertures of 7

mm to 21 mm are similar to apertures seem in downhole televiewer images in the Chalk, and lend

credence to the hypothesis that turbulent flow is a major factor in causing groundwater flooding.

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16:45 Martin Preene.

Preene Groundwater Consulting Ltd.

mp@preene.com

Groundwater problems for construction projects in Chalk.

The role of the Chalk as an important water supply aquifer is well known among hydrogeologists.

However, it is less well understood that the hydrogeological conditions present in the Chalk can

present significant obstacles to tunnel and below ground construction projects. This presentation will

discuss how groundwater problems (and the techniques necessary to overcome them) are largely

controlled by the hydrogeological setting and the degree of weathering of the Chalk. Structureless

chalk or highly weathered structured chalk may exhibit groundwater-related instability comparable to

coarse geotechnical soils such as sands and gravels. Conversely, less weathered structured chalk

may act like massive fissured rock, where large water inflows can occur along pre-existing bedding

planes, fissures and other geological features such as sheet flints. The applicability of construction

dewatering techniques is strongly influenced by the hydraulic conductivity, which can be related

indirectly to weathering grade of chalk. The presentation will include brief case histories of examples

of groundwater control in the Chalk, including cases where large scale works were carried out within

the inner Source Protection Zone of public water supply sources, and where construction dewatering

in chalk created the risk of damaging ground settlements in the land around the site.

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DAY 1 Posters: Chalk Groundwater Resources

Edward Wrathmell, Prodeo Agbotui, Jared L. West, Simon H. Bottrell

University of Leeds, Environment Agency

ee08pya@leeds.ac.uk

Characterisation of the East Yorkshire Chalk aquifer using open-well dilution tests

The Chalk is a very important aquifer in the UK that derives its very high transmissivity from a well-

developed network of solutionally-enhanced (karstic) fractures and conduits. Like other fractured

aquifers, identification and characterisation of flow pathways is necessary for source catchment

delineation and effective management and protection of the groundwater resource. The nature and

vertical extent of permeability development within the saturated zone of the aquifer also influences its

response to extreme events (droughts and groundwater flooding) and contaminant fate and transport.

In this study, open-well dilution testing is applied to existing groundwater monitoring wells in the Chalk

of East Yorkshire. A combination of uniform tracer and point-injection tests have been undertaken in

ambient flow conditions. The results of these single-well tracer tests allow discrete inflow, outflow,

and crossflow zones within the wells to be identified and the direction and rates of flow to be

determined. The results of the flow zone characterisation are combined with stratigraphic information

to confirm that flow is via solutionally enhanced fractures focused at specific horizons, which will vary

according to the topographic position relative to the discharge points. The results inform knowledge

concerning the extent of karst development within the Chalk and its use in groundwater modelling for

aquifer protection and management. The open-well dilution technique used in this study is potentially

applicable for characterising fractured aquifers and hydrocarbon reservoirs worldwide.

Alastair Black, Will Witterick.

Groundwater Science Ltd

a.black@gwscience.co.uk

The future of Chalk groundwater flow modelling?

Substantial changes are in progress in Chalk groundwater modelling. With migration

from the UK adopted Modflow 1996 to the 2017 official release we have the ability –

finally – to represent detailed Chalk stratigraphy, weathering, development and

hydrogeologically significant surfaces without the penalties that led to former

geological oversimplification. Here we show a groundwater model which is

representative of the detailed Chalk geological model. In addition, historic UK led

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innovations in Chalk groundwater flow, recharge and water accounting processes

have been ported into the new paradigm. These collective developments are in

active use in several ongoing studies. This is where we believe the Chalk

groundwater modelling community is headed.

Andy Farrant, Lou Maurice, Marianne Stewart, Illias Karapanoc, Alister Leggatt. BGS

arf@bgd.ac.uk

Assessing the impact of karst recharge in Chalk groundwater catchment, Chiltern Hills, UK

The Chalk is a carbonate aquifer and is thus prone to the same dissolutional processes as other

limestone aquifers. The presence of karst features including sinking streams, sinkholes, conduits and

sediment filled dissolution pipes also means that the Chalk may be prone to rapid groundwater flow

with reduced contaminant attenuation. Tracer tests often indicate rapid flow through times from

stream sinks to springs, with travel times far faster than predicted from groundwater models. When

assessing the potential risk to water supply boreholes from contaminants (nitrates and pesticides), the

potential for the injection of recharge deep into the aquifer via point-source stream sinks and

sinkholes, and rapid karstic flow to boreholes and springs needs to be considered. Using a

combination of tracer test data, geology, land-use information and hydrological characterization of

stream sinks we have assessed the potential risk to public water supply boreholes from point source

karstic recharge for a suite of catchments in the Chiltern Hills. Using a ‘source to sink’ approach to

catchment management, we identified sites that are prone to the potential rapid input and transport of

contaminants. Using this approach enables appropriate, targeted and cost effective remediation

strategies such as land use changes to be put into place where it is likely to have the greatest impact.

Aidan Foley, Hanna Landquist

EGG

aidan.foley@eggconsult.co.uk

Quantifying the geomorphic relationship between Chalk dolines and overlying Palaeogene and Quaternary cover and its significance for groundwater vulnerability.

Dolines represent surface expression of subsurface karst processes operative within limestone

aquifers. Within the English Chalk, dolines have been frequently noted in relation to allochthonous

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point-source recharge from runoff generated on overlying Palaeogene clay or Quaternary Clay-with-

Flints cover. As the Palaeogene cover has retreated over geological time, dolines formed by such

concentrated recharge at the boundary between the Chalk and the clays have been exposed to

subaerial weathering for longer periods of time with distance from that boundary. Hence, fewer of

them are apparent at the surface with increasing distance (Maurice et al, 2006). This poster presents

results from the quantification of this relationship, which has been made possible by the development

of a new solution features database for the Chalk. Different types of doline and other geo- and

anthropomorphic features of the Chalk and overlying deposits will be presented. As dolines represent

karst, and thus enhanced groundwater vulnerability, the knowledge of their distribution may help

inform groundwater vulnerability mapping and modelling.

Alex Gallagher

WSP

alex.gallagher@wsp.com

A review of geological controls on groundwater flow in the Chalk of the UK

Geological processes from the Cretaceous to the Quaternary have shaped the Chalk aquifer and

today exert a strong influence on groundwater flow. Erosion during episodic periglaciation and sea

level change in the Quaternary removed much of the Palaeogene cover, established new base levels

and stimulated recharge and groundwater flow in the Chalk. The effects may be observed in the

distribution of karst features at surface and dissolution at surprising depth. The rising and falling of

Cretaceous sea levels altered the depositional landscape over relatively short periods of time. Marl

horizons were deposited in deeper waters and hardgrounds formed during marine recessions. In

addition, silica rich beds of flint contrast with the soft chalk. Wherever there are contrasting rock

properties there is increased potential for fracturing and enhanced groundwater flow. Work by

Maurice et al., (2012) demonstrated an average fracture spacing of 5-6m in the Chalk of Berkshire. All

but a two of 17 flow horizons observed with borehole imaging in the study were associated with

distinct lithological features. Heterogeneity controls groundwater flow in the unsaturated zone, also.

Gallagher et al., (2012) showed that the wetting up of the unsaturated zone and the onset of drainage

is related to the interaction between hydraulic barriers, such as marls, and the fracture network. It is

well established that fracturing decreases with depth and that transmissivity increases towards

discharge zones. However, discrete horizons can buck this trend. Careful consideration of the

geological environment and investigation methods is required for optimum conceptual understanding.

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Mark Grout

Environment Agency

mark.grout@environment-agency.gov.uk

The role of numerical groundwater models in the management of groundwater resources within the chalk aquifer system of East Anglia, UK.

Some twenty years ago, as part of the regulator-led, national initiative to develop a framework of

numerical groundwater models covering the main aquifer systems across England and Wales, the

former Anglian Region of the Environment Agency published its regional groundwater investigations

and modelling strategy. The strategy led to the development of a coordinated set of numerical

groundwater models covering the aquifer systems of East Anglia, Essex and Lincolnshire. The

programme to develop the models was finally completed in 2015. Over the last 10 years, the models

have been used to underpin decision-making with regard to the Habitats Directive Review of

Consents, the Restoring Sustainable Abstraction Programme, the WFD Investigations Programme,

and by water companies to take forward work under their National Environment Programmes. The

models have been used for abstraction licensing, both in terms of shaping licensing policy and

abstraction licence determination. The models will be used in the coming years to support the

development of a reformed abstraction licensing system. The models provide the only tool which

allows the Environment Agency to assess the long term impacts of all abstractions acting in

combination, and abstracting beyond historical levels up to the fully licensed level of abstraction, and

they provide a powerful tool for appraising abstraction options. The models have provided a common

platform for water resources and water quality investigations; most notably they have been used by

the former Anglian Region to re-define source protection zone boundaries.

Harris Tarnanas, James Senior

Environment Agency

harris.tarnanas@environment-agency.gov.uk

Using Numerical modelling to Assess Groundwater Resources

The Chalk aquifer of East Yorkshire is extensively utilised for groundwater abstraction for numerous

uses including both public and private water supply, agriculture and industry. It is classified a

principal aquifer by the Environment Agency for supporting water supply and surface water baseflow

on a strategic scale.

Numerical modelling allows us to set up and simulate different water supply and demand scenarios in

the aquifer, assess the Chalk’s water balance and its interaction with surface water bodies. The EA

uses the specialised system NGMS to set up these scenarios and visualize the results of numerical

modelling.

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To protect groundwater resources in the Chalk principal aquifer, the EA reviews new abstraction

applications using a recently updated regional distributed numerical groundwater model that has been

uploaded on NGMS.

The model allows us to create and place a new abstraction in the Chalk and to have a look at the

effects on groundwater levels, groundwater flows and surface water flows. We can then view the

impacts of a new abstraction under conditions based on recent actual anthropogenic activity on the

water environment or under worst case scenario conditions where water demand is at the maximum.

By using the model we can better understand the impacts to both groundwater and surface waters in

hydraulic connection with the aquifer and inform applicants at an early stage as to whether they need

to have restrictions in place, as part of an abstraction licence, to protect a groundwater dependant

habitat.

Jo Thorp, Chris Rowell, Mark Lee, Ruth Finlayson

Atkins, Skanska

jo.thorp@atkinsglobal.com

River Humber Gas Pipeline Replacement Project – working collaboratively to prevent saline intrusion during construction dewatering in the northern province Chalk.

Humber Pipeline Tunnel Joint Venture are working on behalf of National Grid to construct a tunnel

under the River Humber for a replacement high pressure gas pipeline. The tunnel is being driven by

tunnel boring machine (TBM) through the Northern Chalk aquifer from the south of the Humber at

Goxhill, North Lincolnshire under the Estuary to Paull, East Yorkshire. Dewatering of the Chalk is

necessary at the construction sites on both sides of the Humber to allow the construction of the TBM

drive pit at Goxhill and reception shaft at Paull.

Dewatering at Goxhill presented a challenge in terms of aquifer protection, as the 12 m drawdown

needed for construction had to be achieved without risking saline intrusion from the adjacent Humber

Estuary into a region of the Lincolnshire Chalk that is heavily abstracted both for public water supply

and private agricultural and domestic supplies. Through close collaboration between HPT JV, the

Environment Agency, Atkins, WJ Groundwater and National Grid, an abstraction-recharge dewatering

system was developed. This limited the influence of dewatering to within a sheet piled cofferdam

surrounding the drive pit and caused minimal drawdown in the surrounding aquifer. The associated

monitoring regime allowed groundwater quality in the aquifer between the drive pit and the Humber

Estuary to be closely scrutinised throughout the works so that any changes that would indicate saline

intrusion could be swiftly identified and mitigated. Close collaboration was maintained between all

parties throughout the works, and by managing the scheme under a groundwater investigation

consent the local Environment Agency team were able to maintain a direct involvement. Construction

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of the drive pit was completed in January 2018 without causing any identifiable impacts to

groundwater quality or levels in the wider aquifer or to nearby receptors.

Geert-Jan Vis, Armin Menkovic, Ronald Vernes.

TNO

geert-jan.vis@tno.nl

The geology of the Chalk in the Netherlands

Roughly 85% of the Dutch on and offshore territory contains Cretaceous Chalk of some sort. Of that

area, some 0.01% occurs at depths within a few (tens of) meters from the land surface in the south

east of the country (South Limburg). Interest in the Chalk in the Netherlands is mainly from an

hydrocarbon exploration point of view (two producing oil fields), and from a groundwater extraction

point of view. Although the majority of the Netherlands is covered by either 3D or 2D seismic surveys,

the 0.01% of Chalk in the south east lies in one of the most poorly seismically imaged areas.

Therefore, the mapping of the Chalk is done using borehole data, supplemented with outcrop and

quarry data. During the last decades, geological mapping activity in the area was relatively low profile.

Recently, interest is growing due to demands for more detailed and accurate geological models which

form the basis for updates of the geohydrological models. A new cored borehole which is currently

being drilled will be used for new analyses on properties such as porosity, permeability, carbonate

content and mineralogy. Next to that, the sedimentology and tectonic history of the area will studied

using data from the new borehole and from existing data in our database. The aim is to update the

geological and geohydrological models in the coming years.

Maria Walentek

Royal HaskoningDHV

maria.walentel@rhdhv.com

The impact of public water supply abstraction from the chalk aquifer on flow and ecological potential of the river Wandle.

The River Wandle is a chalk river in south London which emerges from a spring line at the foot of the

North Downs and flows due north to its confluence with the River Thames. The upper reaches of the

Wandle are split into two branches, the Carshalton Branch and the Croydon Branch.

Since flow in the River Wandle under natural conditions is supported primarily by groundwater, it is

likely that abstraction from the Chalk aquifer will have had some impact on the balance of

groundwater flow that passes underneath the London Clay and therefore also that which emerges as

surface water at Carshalton ponds.

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The Environment Agency has determined that under the WFD classification system, the ecological

potential of the River Wandle Carshalton Branch is currently ‘poor’, and has set a target of achieving

‘Good’ Ecological Potential (GEP) by the year 2027. A NEP project was initiated by SESW to

investigate the potential impact of groundwater abstraction on river flow, and determine whether this

is impacting the ecological potential of the river.

Royal HaskoningDHV was commissioned in 2011 by SESW to lead the 4-year investigation. We have

been involved in all aspects of project planning and delivery including desk-based assessments,

fieldwork, analysis and stakeholder consultation.

Through our investigation and expert assessment, Royal HaskoningDHV has helped establish, and

quantify the potential impacts of groundwater abstraction on river flows.